1. Field of the Invention
The present invention relates to a disk drive using a disk as a recording medium, and more particularly to an apparatus and method for driving a spindle motor to rotate the disk, having a low power consumption.
2. Description of the Related Art
In general, disk drives using disks as a recording medium utilize a motor, called a spindle motor, to rotate the disks at high speed. Spindle motors are brushless DC motors. The power supply voltage for disk drives is directly used as that for the spindle motor. The power supply voltage for disk drives is generally a high voltage sufficient to reliably drive the spindle motor. However, if the spindle motor is powered by a high voltage for a long period of time, the power consumption increases.
The disk drives used in portable personal computers run on batteries, and recently, battery-powered radio-linked disk drives have become available. The power consumption of these types of disk drive is thus required to be reduced, if they are designed to be used for long periods of time.
To this end, Japanese Patent Application KOKAI Publication No. 8-45175 (hereinafter referred to as a prior art document) describes a technique (hereinafter referred to as prior art) for reducing the power consumption of a disk drive (in particular, the power consumption of motor driving means in the disk drive). In the prior art, two power supplies (power supply means) of different voltages, and power selection means are employed. The power selection means selects one of the power supplies in accordance with the rotational speed of the motor for rotating disks, the torque of the motor, or a change in the rotational speed of the motor, thereby causing the selected power supply to supply power to the motor driving means. Specifically, to rotate the motor at 2400 rpm (revolutions per minute), a 12V power supply is selected. On the other hand, to rotate the motor at 600 rpm, a 5V power supply is selected. Further, if the torque of the motor needed to rotate a disk with a head out of contact with the disk is 20 g·cm, the 5V power supply is selected. On the other hand, if the torque of the motor needed to rotate the disk with the head kept in contact with the disk is 80 g·cm, the 12V power supply is selected. Furthermore, in a disk drive in which switching of rotational speeds is needed to be accomplished quickly, a high-voltage power supply is selected for quick switching. Thus, in the prior art, the power consumption of the disk drive is reduced by switching the power supplies in accordance with the rotational speed of the motor for rotating disks, the torque of the motor, or a change in the rotational speed of the motor.
As stated above, the prior art needs at least two power supplies of different voltages. Further, there is a recent tendency to reduce the voltage of the power supply for the disk drive, thereby reducing the power consumption of the entire disk drive. However, if the voltage of the power supply for the disk drive is reduced, it is difficult to, for example, start up the motor when the disk drive is turned on. Nothing about this problem and its solving means is referred to in the prior art document.